A Spitzer/IRAC Characterization of Galactic AGB and RSG Stars

Abstract: We present new Spitzer/IRAC observations of 55 dusty Long Period Variables (LPVs, 48 AGB and 6 RSG stars) in the Galaxy that have different chemistry, variability type, and mass-loss rate. O-rich AGB stars (including intrinsic S-type) tend to have redder [3.6]-[8.0] colors than carbon stars for a given [3.6]-[4.5] color due to silicate features increasing the flux in the 8.0 {\mu}m IRAC band. For colors including the 5.8 {\mu}m band, carbon stars separate into two distinct sequences, likely due to a variable photospheric C$_3$ feature that is only visible in relatively unobscured, low mass-loss rate sources. Semiregular variables tend to have smaller IR excess in [3.6]-[8.0] color than Miras, consistent with the hypothesis that semiregular variables lose mass discontinuously. Miras have redder colors for longer periods while semiregular variables do not. Galactic AGB stars follow the period-luminosity sequences found for the Magellanic Clouds. Mira variables fall along the fundamental pulsation sequence, while semiregular variables are mostly on overtone sequences. We also derive a relationship between mass-loss rate and [3.6]-[8.0] color. The fits are similar in shape to those found by other authors for AGBs in the LMC, but discrepant in overall normalization, likely due to different assumptions in the models used to derive mass-loss rates. We find that IR colors are not unique discriminators of chemical type, suggesting caution when using color selection techniques to infer the chemical composition of AGB dust returned to the ISM.